13.5 Impersonators and Chaperones

An impersonator is a wrapper for a value where the wrapper redirects certain of the value’s operations. Impersonators apply only to procedures, structures for which an accessor or mutator is available, structure types, hash tables, vectors, and boxes. An impersonator is equal? to the original value, but not eq? to the original value.

A chaperone is a kind of impersonator whose refinement of a value’s operation is restricted to side effects (including, in particular, raising an exception) or chaperoning values supplied to or produced by the operation. For example, a vector chaperone can redirect vector-ref to raise an exception if the accessed vector slot contains a string, or it can cause the result of vector-ref to be a chaperoned variant of the value that is in the accessed vector slot, but it cannot redirect vector-ref to produce a value that is arbitrarily different from the value in the vector slot.

A non-chaperone impersonator, in contrast, can refine an operation to swap one value for any another. An impersonator cannot be applied to an immutable value or refine the access to an immutable field in an instance of a structure type, since arbitrary replacement of an operation’s value amounts to mutation of the impersonated value.

Beware that each of the following operations can be redirected to arbitrary procedure through impersonators on the operation’s argument—assuming that the operation is available to the creator of the impersonator:

• a structure-field accesor

• a structure-field mutator

• a structure type property accessor

• application of a procedure

• unbox

• set-box!

• vector-ref

• vector-set!

• hash-ref

• hash-set

• hash-set!

• hash-remove

• hash-remove!

Derived operations, such as printing a value, can be redirected through impersonators due to their use of accessor functions. The equal?, equal-hash-code, and equal-secondary-hash-code operations, in contrast, may bypass impersonators (but they are not obliged to).

In addition to redirecting operations that work on a value, a impersonator can include impersonator properties for an impersonated value. An impersonator property is similar to a structure type property, but it applies to impersonators instead of structure types and their instances.

Programs and libraries generally should avoid impersonator? and treat impersonators the same as non-impersonator values. In rare cases, impersonator? may be needed to guard against redirection by an impersonator of an operation to an arbitrary procedure.

Programs and libraries generally should avoid chaperone? for the same reason that they should avoid impersonator?.

For values that include no impersonators, v1 and v2 can be considered impersonators of each other if they are equal?.

Otherwise, all impersonators of v2 must be intact in v1, in the sense that parts of v2 must be derived from v1 through one of the impersonator constructors (e.g., impersonate-procedure or chaperone-procedure).

See also prop:impersonator-of.

For values that include no chaperones, v1 and v2 can be considered chaperones of each other if they are equal?, except that the mutability of vectors and boxes with v1 and v2 must be the same.

Otherwise, all chaperones of v2 must be intact in v1, in the sense that parts of v2 must be derived from v1 through one of the chaperone constructors (e.g., chaperone-procedure).

13.5.1 Impersonator Constructors

The arity of wrapper-proc must include the arity of proc. The allowed keyword arguments of wrapper-proc must be a superset of the allowed keywords of proc. The required keyword arguments of wrapper-proc must be a subset of the required keywords of proc.

For applications without keywords, the result of wrapper-proc must be either the same number of values as supplied to it or one more than the number of supplied values, where an extra result is supplied before the others. The additional result, if any, must be a procedure that accepts as many results as produced by proc; it must return the same number of results. If wrapper-proc returns the same number of values as it is given (i.e., it does not return a procedure to impersonator proc’s result), then proc is called in tail position with respect to the call to the impersonator.

For applications that include keyword arguments, wrapper-proc must return an additional value before any other values but after the result-impersonating procedure (if any). The additional value must be a list of replacements for the keyword arguments that were supplied to the impersonator (i.e., not counting optional arguments that were not supplied). The arguments must be ordered according to the sorted order of the supplied arguments’ keywords.

Pairs of prop and prop-val (the number of arguments to procedure-impersonator must be even) add impersonator properties or override impersonator-property values of proc.

If any prop is impersonator-prop:application-mark and if the associated prop-val is a pair, then the call to proc is wrapped with with-continuation-mark using (car prop-val) as the mark key and (cdr prop-val) as the mark value. In addition, if continuation-mark-set-first with (car prop-val) produces a value for the immediate continuation frame of the call to the impersonated procedure, the value is also installed as an immediate value for (car prop-val) as a mark during the call to wrapper-proc (which allows tail-calls of impersonators with respect to wrapping impersonators to be detected within wrapper-proc).

The protocol for a redirect-proc depends on the corresponding orig-proc:

• A structure-field accessor: redirect-proc must accept two arguments, v and the value field-v that orig-proc produces for v; it must return a replacement for field-v. The corresponding field must not be immutable.

• A structure-field mutator: redirect-proc must accept two arguments, v and the value field-v supplied to the mutator; it must return a replacement for field-v to be propagated to orig-proc and v.

• A property accessor: redirect-proc uses the same protocol as for a structure-field accessor. The accessor’s property must have been created with 'can-impersonate as the second argument to make-struct-type-property.

Pairs of prop and prop-val (the number of arguments to impersonate-struct must be odd) add impersonator properties or override impersonator-property values of v.

The ref-proc must accept vec, an index passed to vector-ref, and the value that vector-ref on vec produces for the given index; it must produce a replacement for the value, which is the result of vector-ref on the impersonator.

The set-proc must accept vec, an index passed to vector-set!, and the value passed to vector-set!; it must produce a replacement for the value, which is used with vector-set! on the original vec to install the value.

Pairs of prop and prop-val (the number of arguments to impersonate-vector must be odd) add impersonator properties or override impersonator-property values of vec.

The unbox-proc must accept bx and the value that unbox on bx produces index; it must produce a replacement value, which is the result of unbox on the impersonator.

The set-proc must accept bx and the value passed to set-box!; it must produce a replacement value, which is used with set-box! on the original bx to install the value.

Pairs of prop and prop-val (the number of arguments to impersonate-box must be odd) add impersonator properties or override impersonator-property values of bx.

The ref-proc must accept hash and a key passed hash-ref. It must return a replacement key as well as a procedure. The returned procedure is called only if the returned key is found in hash via hash-ref, in which case the procedure is called with hash, the previously returned key, and the found value. The returned procedure must itself return a replacement for the found value.

The set-proc must accept hash, a key passed to hash-set! or hash-set, and the value passed to hash-set! or hash-set; it must produce two values: a replacement for the key and a replacement for the value. The returned key and value are used with hash-set! or hash-set on the original hash to install the value.

The remove-proc must accept hash and a key passed to hash-remove! or hash-remove; it must produce the a replacement for the key, which is used with hash-remove! or hash-remove on the original hash to remove any mapping using the (impersonator-replaced) key.

The key-proc must accept hash and a key that has been extracted from hash (by hash-iterate-key or other operations that use hash-iterate-key internally); it must produce a replacement for the key, which is then reported as a key extracted from the table.

The hash-iterate-value, hash-map, or hash-for-each functions use a combination of hash-iterate-key and hash-ref. If a key produced by key-proc does not yield a value through hash-ref, then the exn:fail:contract exception is raised.

Pairs of prop and prop-val (the number of arguments to impersonate-hash must be odd) add impersonator properties or override impersonator-property values of hash.

The property value must be a procedure of one argument, which is a structure whose structure type has the property. The result can be #f to indicate the structure does not represent an impersonator, otherwise the result is a value for which the original structure is an impersonator (so the original structure is an impersonator-of? and equal? to the result value). The result value must have the same prop:impersonator-of and prop:equal+hash property values as the original structure, and the property values must be inherited from the same structure type (which ensures some consistency between impersonator-of? and equal?).

13.5.2 Chaperone Constructors

For applications that include keyword arguments, wrapper-proc must return an additional value before any other values but after the result-chaperoning procedure (if any). The additional value must be a list of chaperones of the keyword arguments that were supplied to the chaperone procedure (i.e., not counting optional arguments that were not supplied). The arguments must be ordered according to the sorted order of the supplied arguments’ keywords.

• With a structure-field accessor as orig-proc, redirect-proc must accept two arguments, v and the value field-v that orig-proc produces for v; it must return a chaperone of field-v. The corresponding field may be immutable.

• With structure-field mutator as orig-proc, redirect-proc must accept two arguments, v and the value field-v supplied to the mutator; it must return a chaperone of field-v to be propagated to orig-proc and v.

• A property accessor can be supplied as orig-proc, and the property need not have been created with 'can-impersonate. The corresponding redirect-proc uses the same protocol as for a structure-field accessor.

• With struct-info as orig-proc, the corresponding redirect-proc must accept two values, which are the results of struct-info on v; it must return each values or a chaperone of each value. The redirect-proc is not called if struct-info would return #f as its first argument.

An orig-proc can be struct-info only if some other orig-proc is supplied, and each orig-proc must indicate a distinct operation. If no orig-procs are supplied, then no props must be supplied, and v is returned unchaperoned.

The struct-info-proc must accept 8 arguments—the result of struct-type-info on struct-type. It must return 8 values, where each is the same or a chaperone of the corresponding argument. The 8 values are used as the results of struct-type-info for the chaperoned structure type.

The make-constructor-proc must accept a single procedure argument, which is a constructor produced by struct-type-make-constructor on struct-type. It must return the same or a chaperone of the procedure, which is used as the result of struct-type-make-constructor on the chaperoned structure type.

The guard-proc must accept as many argument as a constructor for struct-type; it must return the same number of arguments, each the same or a chaperone of the corresponding argument. The guard-proc is added as a constructor guard when a subtype is created of the chaperoned structure type.

Pairs of prop and prop-val (the number of arguments to chaperone-struct-type must be even) add impersonator properties or override impersonator-property values of struct-type.

The proc generator is called on synchronization, much like the procedure passed to guard-evt, except that proc is given evt. The proc must return two values: a synchronizable event that is a chaperone of evt, and a procedure that is used to check the event’s result if it is chosen in a selection. The latter procedure accepts the result of evt, and it must return a chaperone of that value.

Pairs of prop and prop-val (the number of arguments to chaperone-evt must be even) add impersonator properties or override impersonator-property values of evt.

13.5.3 Impersonator Properties

• an impersonator property descriptor, for use with impersonate-procedure, chaperone-procedure, and other impersonator constructors;

• an impersonator property predicate procedure, which takes an arbitrary value and returns #t if the value is an impersonator with a value for the property, #f otherwise;

• an impersonator property accessor procedure, which returns the value associated with an impersonator for the property; if a value given to the accessor is not an impersonator or does not have a value for the property (i.e. if the corresponding impersonator property predicate returns #f), the exn:fail:contract exception is raised.